GUI should not show Kalman filter fields for flow tracker

[talmo]

In #1986, we fixed the Kalman filter that had mismatched args with the new CLI.

We disabled it with the flow tracker since it doesn't make as much sense to use it with the Kalman filter, but its fields are still present in the GUI. These should be removed.

In the simple tracker, we should also have GUI validation to make sure that the number of tracks is set.